Changing the form of electric power. For example, electricity from the grid is transmitted in the form of AC, while electronic circuits need low-level DC;

Regulation. The nominal mains voltage varies worldwide from 100 to 240VAC and is usually poorly regulated, while the circuits normally require well stabilized fixed voltages;

Safety isolation. In most applications the outputs have to be isolated from the input.

Practically every piece of electronic equipment needs some form of power conversion. Power supply unit (PSU), technically speaking, is a device that transfers electric energy from a source to a load and in the process changes its characteristics to meet specific requirements. Of course, this term is not the most adequate. A PSU does not really supply power, it only converts it. Its typical application is to convert a utility's AC into required regulated DC rail(s). Depending on the mode of operation of the semiconductors, the converters can be linear or switching.

SMPS

stands for switch mode PSU. In such a device, power handling electronic components are continuously switching "on" and "off" with high frequency in order to provide the transfer of electric energy via energy storage components (inductors and capacitors). By varying duty cycle, frequency or a relative phase of these transitions an average value of output voltage or current is controlled. The operating frequency range of commercial SMPS units varies typically from 50 kHz to several MHz (see more on frequency selection).
Below is a conceptual circuit diagram of a typical off-line SMPS. This tutorial will introduce you to its basic operation.

DESIGN BASICS

AC power first passes through fuses and a line filter. Then it is rectified by a full-wave bridge rectifier. The rectified voltage is next applied to the power factor correction (PFC) pre-regulator followed by the downstream DC-DC converter(s).

Note that except for some industries, such as PCs and CompactPCI, PSU output connectors and pinouts in general are not standardized and are left up to the manufacturers.

F1 and F2 shown on the left of the circuit diagram are fuses. Everybody knows about them, but some people are under impression that a fuse blows immediately once applied current exceeds its rating.

If that was true, no PSU would function because of momentary "in-rush" currents. In reality, a fuse is designed to physically open the circuit when the current being drawn through it exceeds its rating for a certain period of time. This clearing time depends on the degree of overload and is a function of I2t. Due to this delay, fuses will not always protect electronic components from a catastrophic failure caused by some fault conditions. Their main purpose is to protect the upstream line from overloading and overheating, avoid tripping of an external circuit breaker, and prevent a fire that may be triggered by components that failed into a short circuit.
The low-pass EMI filter is designed to reduce to an acceptable level high frequency currents getting back into the AC line. This is necessary to prevent interference on the other devices connected to the same electrical wiring. There is a number of standards (such as EN55022 for Information Technology equipment) that govern the maximum level of EMI.
The filter is followed by the rectifier that converts bipolar AC waveforms to unipolar pulsating ones. It has four diodes in a bridge arrangement to provide the same polarity of the output for both polarities of the input.

PFC regulator

. The rectified input voltage is fed into the next stage, whose prime purpose is to increase power factor (PF). By definition, PF is the ratio between watts and volt-amps. In the process, the PFC pre-regulator usually boosts the voltage to 370-400 VDC. There are also designs where "boost" DC-link follows the peak of input AC voltage instead of being fixed, or where a buck converter is used instead of a boost.

There are two main types of power factor correction circuits- active and passive. Below is a block-diagram of an active PFC stage. Here is how it works. A controller monitors both the voltage across sense resistor and Vboost. While regulating Vboost, it controls at the same time the shape of the input current, so that it is in phase with mains AC and repeats its waveform. Without this, the current would be delivered to the SMPS in short high level pulses, which have a high harmonic content. The harmonics do not supply any real energy to the load, but cause additional heating in the wiring and distribution equipment. They also reduce the maximum wattage that can be taken from a standard wall outlet, since circuit breakers are rated by electric current rather than by watts. There are various regulations that limit the input harmonic content, such as EN61000-3-2 (for equipment connected to public low-voltage distribution systems) or DO-160 (for airborne equipment). To meet these requirements you can use a PF correction technique: a device with a high PF draws a nearly sinusoidal current from the source (at a sinusoidal input). This automatically results in low harmonic content. Currently there are no mandatory international standards that specifically regulate the PF of an electronic equipment, but there are various national and industry standards as well as voluntary incentive programs. For example, 80 PLUS® and Energy Star® programs require computers to demonstrate PF>0.9 at rated load. You can read about active power factor correction in this PFC guide.
The above standards also specify minimum efficiency of certain classes of electronic devices. The efficiency of a PSU by definition is the ratio between the values of output and input wattage: Efficiency=Pout/Pin. Note that because Pin=VA*PF and since any real active circuit has PF<1, you can't just multiply input volts and amps- to measure Pin you need a true wattmeter.

The downstream DC-DC converter runs off the PFC output, generates a set of DC busses required for the load, and normally also provides input-to-output isolation. There are a number of topologies utilized in DC-DC converters. The above block diagram depicts an isolating forward converter. Most low-voltage non-isolated converters use buck regulators (single or interleaved multi-phase). There is likewise a large variety of PWM ICs suitable for each of these topologies. The selection of the right power topology depends on specific requirements for the product (including cost and time factors).

Finally, the housekeeping supply provides "bias" for all control circuitry. It may also provide a separate stand-by voltage (SBV) which remains active when the PS unit is shut down for any reason. In today's computer power supply a 5VDC SBV is a standard feature.

If you want to learn practical PSU design, you may start with Unitrode seminar books, where you can find a comprehensive collection of power supply tutorials, practical schematic diagrams, and guides.